Evol Ecol (2012) 26:825–845
DOI 10.1007/s10682-011-9538-y
ORIGINAL PAPER
Relationships between head morphology, bite
performance and ecology in two species
of Podarcis wall lizards
Antigoni Kaliontzopoulou • Dean C. Adams • Arie van der Meijden
Ana Perera • Miguel A. Carretero
•
Received: 19 July 2011 / Accepted: 3 November 2011 / Published online: 18 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Understanding the relationship between form and function is central to our
comprehension of how phenotypic diversity evolves. Traits involved in multiple activities,
such as social interactions and ecological resource use, are under the influence of different
evolutionary forces potentially acting in opposite directions. Such systems provide the
opportunity of understanding how potential constraints on morphological variation may
influence whole-organism performance. In this study we examined morphology and bite
performance in two closely related species of Podarcis wall lizards with divergent
microhabitat preferences, to investigate how natural and sexual selection interact to shape
the evolution of head traits. Our results show that although head morphology is markedly
different between species and sexes, only sexes differ in bite force, indicating that the
ecological differentiation between species is reflected in their morphology but does not
constrain performance. Rather, the modification of the relative size of head components
between species and a shift in the form-function relationship provide a potential explanation of how equal performance is attained by different morphological configurations.
Geometric morphometrics provide a clear, biomechanically meaningful image of how this
is achieved and show a bisexual pattern of head shape-bite force association in both
species. This, together with a strong allometry of head size on body size and head shape on
head size, provides indirect morphological evidence for the importance of sexual selection
in shaping morphological and functional patterns. Finally, our findings suggest that the
differences observed between species and sexes in head traits and bite performance are not
reflected in their dietary ecology, implying that if trophic niche segregation between
A. Kaliontzopoulou (&) A. van der Meijden A. Perera M. A. Carretero
CIBIO/UP, Centro de Investigação em Biodiversidade e Recursos Genéticos, Campus Agrario de
Vairão, 4485-661 Vairão, Portugal
e-mail: antigoni@mail.icav.up.pt
A. Kaliontzopoulou D. C. Adams
Department of Ecology, Evolution, and Organismal Biology, Iowa State University,
Ames, IA 50011, USA
D. C. Adams
Department of Statistics, Iowa State University, Ames, IA 50011, USA
123
826
Evol Ecol (2012) 26:825–845
groups occurs, the reasons behind it are not primarily related to head morphology and
functional variation.
Keywords Head shape Geometric morphometrics Sexual dimorphism Allometry Performance
Introduction
The relationship between morphology, performance and ecology is central to our understanding of the evolutionary and ecological processes that drive phenotypic evolution
(Wainwright 2007). External morphology is the result of the amalgamation of historical
factors, natural and sexual selection, but also proximate factors such as physiology, body
condition and growth. Nevertheless, it is widely recognised that performance, rather than
morphology, is most frequently the target of selection (Wainwright 2007; Irschick et al.
2008). This means that performance traits, and their association to morphology and
ecology, are of cornerstone importance for determining the role and relative contribution of
the aforementioned processes in shaping the morphological variation observed in any
organismal system (Arnold 1983; Kingsolver and Huey 2003).
Head morphology of lizards and its relation to bite performance and ecology has been a
major model system for investigating the integration of different phenotypic components.
In many lizard species, the head is involved in multiple and highly relevant ecological and
social activities, including feeding, refuge use, mating and male aggressive interactions.
Indeed, sexual dimorphism in head size and shape is prominent in many species (Cooper
and Vitt 1989; Preest 1994; Braña 1996; Kratochvı́l and Frynta 2002; Kaliontzopoulou
et al. 2008), a pattern that has been attributed to sexual selection enhancing male potential
for antagonism and territoriality (Stamps 1983; Andersson 1994; Braña 1996; Huyghe
et al. 2005). This selective mechanism has been shown to act through influences on bite
force (Lappin and Husak 2005; Lappin et al. 2006, Husak et al. 2006a; Lailvaux and
Irschick 2007; Husak et al. 2009; Herrel et al. 2010). Further, both head morphology and
bite force are highly relevant for feeding (Herrel et al. 1998a, 2001d; Metzger and Herrel
2005; Vincent and Herrel 2007), suggesting that natural selection can also play a role in
head shape and bite performance diversification. Additionally, head dimensions are
directly associated with microhabitat and refuge use (Arnold 1998) and both head size and
shape have been shown to be under the influence of natural selection (Vitt et al. 1997;
Vanhooydonck and Van Damme 1999; Herrel et al. 2001b; Kaliontzopoulou et al. 2010),
under the selective influence of predation. All the above processes act simultaneously,
potentially in opposite directions, and different equilibria between them may cause variation in morphology and performance both within and among lizard species.
In the same context, several studies have shown that sexual and natural selection may
interact in complex ways, the outcome being variation in the degree of sexual dimorphism
in morphological traits across environmental conditions or species (Butler and Losos 2002;
Losos et al. 2003; Butler et al. 2007; Stuart-Fox and Moussalli 2007; Kaliontzopoulou et al.
2010). However, the consequences of such interactions for performance have not been
investigated in detail. As different functional demands cannot always be simultaneously
optimized for the same trait (Perrin and Travis 1992; Walker 2007), the existence of
counteracting forces may lead to varying outcomes, depending on the relative force of
sexual and natural selection, the traits targeted by the selective mechanisms involved and
the strength of dependence among phenotypic traits. For instance, if the optimisation of
123
Evol Ecol (2012) 26:825–845
827
bite performance under the influence of sexual selection is crucial and morphology-performance relationships are relatively flexible, different head morphologies may accommodate similar bite–force outcomes through the modification of head proportions, internal
head anatomy or physiology. In turn, if ecological adaptation to habitat type poses a
powerful constraint on head morphology, bite performance may also be constrained as a
consequence. Systems where ecological components and/or social interactions are known
to vary are ideal for investigating how these different forces interact to shape the evolution
of head morphology, bite performance and the relationship between them.
Several previous studies have analysed the effects of body size, head size and head
shape on bite performance in lizards (Herrel et al. 2001a; Verwaijen et al. 2002; Herrel and
O’Reilly 2006; Lappin et al. 2006; Huyghe et al. 2009; Measey et al. 2009; among others).
These studies have shown that, while general rules apply, extensive variation also exists,
with different morphological variables being more relevant for bite performance in different groups (see for example Herrel et al. 2001a; Lappin and Husak 2005). This is not
unexpected, since bite performance is influenced by several components. The inleveroutlever proportions of the jaw, the size, insertion points and orientation of the jaw
musculature and the micro-structure of muscle fibers jointly determine bite force production (Herrel et al. 1998a, b, 2001c). External head morphology is an easily accessible
proxy of the combined action of all the above structural components. However, with such
diverse factors involved, making sure to record the most relevant morphological parameters can be a challenge and an integrated quantification of morphology is urged. This can
be achieved using geometric morphometric (GM) methods, which provide a powerful
statistical framework for the analysis of organismal shape while preserving the geometric
properties of the studied structures (Bookstein 1991; Adams et al. 2004; Slice 2005). These
tools have been increasingly used to study the morphology of complex structures,
including lizard head shape (Bruner and Constantini 2007; Kaliontzopoulou et al. 2007,
2008, 2010; Ljubisavljević et al. 2010; Raia et al. 2010; among others). GM methods have
also been used successfully to capture biomechanically-relevant shape (Adams and Rohlf
2000; Adams 2004; Rivera 2008; Young et al. 2010), but have not yet been used to
investigate lizard head morphology in relation to bite performance.
In this study, we examined head morphology, bite performance and diet in two species
of Podarcis wall lizards from the Iberian Peninsula with different habitat preferences, to
investigate how different evolutionary forces on morphology and performance interact to
shape head trait evolution. For the first time, we used both linear and geometric morphometrics to capture variation in head morphology and associate it to bite performance
and ecology. Specifically, we aimed at answering the following questions: (1) Are species
and sexes morphologically and functionally differentiated, as we expect based on previous
knowledge about the action of sexual and natural selection on head traits? (2) If so, which
are the morphological traits most highly associated with bite-force variation? (3) Are
relationships between head morphology (size and shape) and bite performance uniform
across species and sexes? (4) Do species and sexes use different dietary resources? Based
on previous studies, we expected to observe the typical patterns of sexual dimorphism
(enlarged head traits and higher bite force in males) and the typical morphological differentiation between species inhabiting divergent microhabitats (saxicolous lizards should
be flatter). We anticipated that the use of GM methods would further augment our
understanding of morphology-performance dimorphism in these lizards; if head shape is
modified in males under the influence of sexual selection to enhance bite performance, we
expect variation in head shape in males to be correlated with variation in bite force, while
such a relationship should be lacking in females. Our comparison between species and
123
828
Evol Ecol (2012) 26:825–845
sexes also provides evidence on the relative importance of sexual versus natural selection
in this phenotypic system. If the influence of natural selection on morphology dominates,
we predict that the flat, saxicolous P. hispanica type 1A males may show decreased bite
performance as compared to the ground-dwelling P. bocagei males. In turn, if the influence
of sexual selection for increasing bite force in males prevails, we may observe a shift in
morphology-performance across species, as a means to maintain important functionality
(bite force) under the different head configurations dictated by microhabitat use. Finally,
given the importance of head morphology and bite performance for feeding, species and
sexes may also use different dietary resources, dietary divergence constituting an additional mechanism that maintains morphological and performance differentiation in this
system.
Materials and methods
Study organisms
We focused on two species of wall lizards, Podarcis bocagei Seoane, 1884 and
P. hispanica (Steindachner 1870) type 1A (sensu Harris and Sá-Sousa 2002). These two
species present a particularly interesting system for studying morphology-function-ecology
relationships, due to their largely shared evolutionary history and geographical distribution
as well as their ecological differentiation. They are sister taxa, both being members of the
P. hispanica species complex (Pinho et al. 2006; Kaliontzopoulou et al. 2011). Additionally, they share a large portion of their geographic distributions (Kaliontzopoulou et al.
2011) and are frequently found in strict syntopy, where similar ecological resources are
available to both (Carretero 2008). However, they are quite distinct ecologically:
P. bocagei is primarily ground-dwelling, but also inhabits human-constructed walls
(Kaliontzopoulou et al. 2010), while P. hispanica type 1A is primarily saxicolous, being
more restricted to big rock outcrops and frequently climbing perpendicular surfaces
(Sá-Sousa et al. 2002). Our study was conducted in one of these sympatric sites, found on a
coastal location in northern Portugal (Moledo: 41°50.3140 N, 8°41.8390 W). In this site, both
species are principally found on agricultural walls, where they share large part of their
microhabitat. We visited the study site in June 2009 and collected a total of 32 male and 29
female P. bocagei and 33 male and 30 female P. hispanica type 1A adult individuals. At
the moment of capture, fecal pellets were obtained from all individuals and preserved in
70% alcohol for diet composition analysis. All lizards were then taken to the laboratory
and housed there for 3 days in order to conduct bite force measurements and to quantify
morphology. Lizards were housed in a single, 1 m 9 1 m terrarium, with natural substrate.
The terrarium was kept under natural light-cycle conditions and provided with access to
direct sunlight for at least 8 h per day (including the hours during which all bite-measurements took place, see below). After performance experiments (see below) all animals
were released back to their location of capture.
Quantified parameters
To quantify morphological variation relevant to bite force and prey consumption we
measured four head characters to the closest 0.01 mm using electronic calipers: head
length (HL), from the tip of the snout to the posterior border of the collar, head width (HW)
at the widest point of the head, head height (HH) at the highest point and mouth opening
123
Evol Ecol (2012) 26:825–845
829
(MO), from the tip of the snout to the posterior border of the last supralabial scale (Fig. 1).
We also measured snout-vent length (SVL) to represent total body size. All linear variables
were log-transformed prior to analyses. Additionally, we obtained high resolution photographs of the right side of the head to study lateral head shape and used tpsDig software
(Rohlf 2005) to digitize 12 landmarks (Fig. 1) on the head of each specimen. We then used
geometric morphometric approaches (Rohlf and Marcus 1993; Adams et al. 2004) to
compare patterns in head shape. First, we performed a Generalized Procrustes Analysis
(GPA: Rohlf and Slice 1990) to superimpose all specimens to a common location, and
remove the effects of size, location and orientation from the landmark coordinates. We
then obtained partial warp scores and uniform shape components (using tpsRelW: Rohlf
2008), which were treated as a set of shape variables for subsequent statistical analyses.
Bite force was measured using an isometric Kistler force transducer (type 9203, Kistler
Inc., Winterthur, Switzerland) mounted on a vertical holder and connected to a Kistler
charge amplifier (type 5058A, Kistler Inc., Winterthur, Switzerland). Bite force measurements were obtained by provoking the lizards to bite a pair of thin metal plates
connected to the force transducer (see Herrel et al. 2001d for a detailed description). The
tip of the metal plates where the lizards bit was delimited with a marker to ensure all
lizards bit at an equal distance from the revolving arms and thus standardize the point of
force exertion. Each lizard was tested five times to ensure that the maximal individual bite
force per individual was registered. Before performing bite tests, lizards were allowed to
thermoregulate under direct natural sunlight and in a room temperature of about 25°C to
ensure that activity body temperatures were attained. The maximum bite force measure per
individual was retained and log-transformed for further analyses.
We characterized diet composition for each individual by examining preserved fecal
pellets through a binocular dissecting microscope. Prey items were identified to the taxonomic level of Order (all observed prey were arthropods, see ‘‘Results’’), with the
exception of Formicidae, which was considered separately from other Hymenoptera due to
their non-flying, aggregated nature. Prey counts were based on cephalic capsules, wings
and legs, following the minimum numbers criterion per sample (Carretero 2004). The
length and width of intact arthropod body structures (Coleoptera elytra, Hymenoptera and
Diptera wings and Homoptera and Heteroptera hemielytra) were measured to the nearest
0.5 mm, which served as a proxy for the size of the consumed prey items. Apart from
taxonomic classification, prey were also classified into functional categories based on their
Fig. 1 Linear measurements taken in the head of the lizards and landmarks digitized for quantifying lateral
head shape using geometric morphometrics. HL head length, HW head width, HH head height, MO mouth
opening
123
830
Evol Ecol (2012) 26:825–845
hardness and mobility, which could potentially be more informative in ecological terms
from the point of view of lizard foraging. Three hardness classes were distinguished based
on previous hardness measurements obtained through prey-crushing (Herrel et al. 2001d;
Verwaijen et al. 2002): adults of Araneae, Diptera, Hymenoptera, Lepidoptera, Mecoptera
and Zygentoma, as well as all larvae, were considered as soft prey; adults of Dermaptera,
Formicidae, Heteroptera and Homoptera were considered as intermediate-hardness prey;
and adults of Coleoptera and Gastropoda were considered as hard prey. Based on their
mobility (Perry and Pianka 1997), prey items were classified into sedentary (Dermaptera,
Gastropoda and Heteroptera), walking/jumping (Araneae, Coleoptera, Formicidae,
Homoptera, Zygentoma and all larvae) and flying categories (Diptera, Hymenoptera,
Lepidoptera and Mecoptera).
Statistical analyses
To represent total head size (HS) we used Mosimann’s (1970) geometric mean approach
and calculated HS as the third root of the product of HL, HW and HH. We tested for intraand interspecific differentiation in the quantified morphological traits using factorial univariate and multivariate analysis of variance ((M)ANOVA), with species (SP), sex (SEX),
and their interaction (SP 9 SEX) as model effects. To gain more insight into potential
allometric variation in morphological traits, we performed regression analyses between
body size (SVL) and head size (HS), between all head dimensions and head size, and
between lateral head shape and head size. We also included SP and SEX, as well as all the
interaction terms, in the linear model, in order to test for differences in slopes and intercepts between groups.
To obtain a first idea of the morphological traits potentially relevant for bite force, we
performed ANCOVA on bite force with SP and SEX as factors, and with each of the linear
morphological traits (SVL, HS, head dimensions) treated separately as covariates. All
interaction effects were also included. We then used model selection techniques (Burnham
and Anderson 2002) to evaluate different models based on information theory (Akaike’s
Information Criterion) and to select the combination of linear body measurements that best
explained variation in bite force. The relative importance of the different linear morphological variables was found as the sum of Akaike weights for that variable in all models
with Delta AIC \ 4 (Burnham and Anderson 2002). This procedure was performed on the
pooled data, irrespective of species and sex, and in the four SP 9 SEX groups separately,
to examine whether the importance of different morphological traits for determining bite
force varied across groups. Finally, to assess how variation in lateral head shape was
associated with variation in bite force across groups, we built a linear model of head shape
on SP, SEX, bite force and all interaction effects. To visualize the interdependence
between bite force and lateral head shape, we projected the shape data on the vector
defined by the regression model to obtain individual regression scores that provide a
graphical means to examine the strength of association between a multivariate response
(shape) and a univariate predictor (bite force) (Drake and Klingenberg 2008; Adams and
Nistri 2010).
Diet composition was characterized for each species-by-sex group by implementing the
use index (UI). This index describes the importance of each prey type in the pellets
examined by combining the percentage of pellets containing a given prey type with the
percentage of its numeric abundance. The UI thus represents the homogeneity of consumption of a certain prey type throughout all pellets (see Carretero 2004 and references
therein). We then compared use indices between sexes of the same species (sexual dietary
123
Evol Ecol (2012) 26:825–845
831
disparity) and between species of the same sex (across-species disparity) using the
graphical approach proposed by Carretero and Llorente (2001). We used Pianka’s overlap
index (Pianka 1973) to obtain a quantitative measure of similarity between groups (group
disparity then being 1-overlap). As a complementary approach, we investigated whether
the functional properties of prey (hardness and mobility) influenced their consumption by
different SP 9 SEX groups using a Chi-squared test. We also tested whether SP 9 SEX
groups differed in the size of consumed prey items, by concentrating on the most frequently used prey types, for which sufficient sample sizes were available. Here we calculated the total size of each prey item as the geometric mean of its length and width (both
log-transformed) and then used a non-parametric ANOVA design based on 1,000 permutations to evaluate differences between SP 9 SEX groups in the size of consumed prey
items. Finally, we performed two-way Mantel tests (Legendre and Legendre 1998) to test
whether group differences in morphology (SVL, HS, head dimensions, lateral head shape)
and bite force on one side and diet on the other where associated with each other. The
Euclidean distance matrix was used for morphology and bite force, while the niche
‘‘distance’’ matrix (1—niche overlap) represented diet differentiation between groups.
All statistical analyses were conducted using R v. 2.12.1 (R Development Core Team
2010). Spline graphs representing differences between groups in geometric head shape
were produced using Morpheus et al. software (Slice 1999), while deformation grids of
variation across the bite-force variation range for different groups were produced using
tpsRegr (Rohlf 2009).
Results
Morphological and performance differentiation
Analyses of morphology using (M)ANOVA indicated significant morphological differences between species and between the sexes, and in some cases significant interactions
between factors (Table 1). Males of P. bocagei were significantly larger than conspecific
females (Tukey HSD, P = 0.02), but no significant sexual dimorphism in total body size
(e.g. logSVL) was observed in P. hispanica type 1A (Tukey HSD, P = 0.67) (Fig. 2a).
Individuals of different species but of the same sex did not differ in total body size (Tukey
HSD, P [ 0.05 in both cases). A more prominent differentiation was observed across
groups for HS, which differed significantly between all pairs except between females of the
two species (Tukey HSD, P = 0.16). Males of both species had visibly larger heads than
conspecific females, and males of P. bocagei had larger heads than those of P. hispanica
type 1A (Table 1, Fig. 2b). Head dimensions all differed significantly between sexes, but
not between species, with the exception of head height (HH). In all cases, male lizards had
larger head dimensions than conspecific females. For head height (HH), all model terms
were significant (Table 1) and post hoc comparisons showed that both species and sexes
were significantly differentiated, where males had higher heads than females and individuals of P. bocagei had higher heads than those of P. hispanica type 1A (Fig. 2c).
Lateral head shape also differed between species and between the sexes (Table 1). Thinplate spline deformation grids indicated that P. hispanica type 1A individuals of both sexes
had relatively elongated but flattened lateral heads as compared to those of P. bocagei
(Fig. 3a, b), while the males of both species displayed a relative expansion of the tympanic
area (i.e. the area covered by landmarks 7–11, see Fig. 1), having relatively more robust
heads as compared to females (Fig. 3c, d).
123
832
Evol Ecol (2012) 26:825–845
Table 1 Descriptive statistics and (M)ANOVA results for body size (SVL), linear head measurements,
head size (HS), lateral head shape and bite force. All morphological traits are in mm and bite force readings
in N (all at a logarithmic scale). Degrees of freedom are 3 (design) and 120 (residuals) in all comparisons.
Values shown are the mean ± SE (top) and range (bottom). F-(top) and P-values (bottom) correspond to
analysis of variance comparisons considering the effect of species (SP), sex and their interaction
(SP 9 SEX). All P-values presented were corrected for multiple testing using the false discovery rate
procedure. Significant effects are marked in bold letter
PBF
(N = 29)
logSVL
logHS
PBM
(N = 32)
PHF
(N = 30)
PHM
(N = 33)
SP
1.69 ± 0.01
1.72 ± 0.01
1.7 ± 0.01
1.71 ± 0.01
F
0.07
8.25
1.69
1.63–1.78
1.62–1.77
1.61–1.78
P
0.79
0.005
0.20
0.9 ± 0.01
0.99 ± 0.01
0.88 ± 0.01
0.95 ± 0.01
F
15.98
0.89–1.05
0.82–0.93
0.86–1.02
P
<0.001
logHL
1.17 ± 0.01
1.25 ± 0.01
1.17 ± 0.01
1.24 ± 0.01
F
1.13
1.06–1.24
1.15–1.32
1.1–1.23
1.15–1.34
P
0.29
logHW
0.79 ± 0.01
0.89 ± 0.01
0.78 ± 0.01
0.87 ± 0.01
F
5.35
0.7–0.89
0.78–0.99
0.71–0.83
0.74–0.94
P
0.02
logHH
0.66 ± 0.01
0.75 ± 0.01
0.61 ± 0.01
0.68 ± 0.01
F
66.21
0.56–0.75
0.67–0.83
0.54–0.69
0.58–0.75
P
<0.001
logMO
0.97 ± 0.01
1.06 ± 0.01
0.95 ± 0.01
1.02 ± 0.01
F
10.55
0.88–1.04
0.95–1.12
0.87–0.99
0.93–1.08
P
GM
shapea
a
SP 9 SEX
1.59–1.78
0.81–0.97
logBITE
SEX
133.24
<0.001
100.19
<0.001
134.04
<0.001
103.63
<0.001
125.30
0.83
0.36
0.47
0.49
0.02
0.90
2.27
0.13
1.23
<0.001
0.27
F
15.57
12.28
0.89
P
<0.001
<0.001
10.63
0.11 ± 0.03
0.47 ± 0.03
0.02 ± 0.01
0.36 ± 0.03
F
-0.17–0.37
0.00–0.70
-0.22–0.18
0.00–0.59
P
0.002
0.002
155.34
<0.001
0.60
0.00
0.98
GM shape: lateral head shape as quantified using geometric morphometrics
Finally, bite force was significantly different between the sexes in both species
(P \ 0.01 for both species), but within each sex there were no differences between species
(P [ 0.05 in both sexes). In both species, males attained a higher maximal bite force than
did females (Table 1, Fig. 2d).
Allometric patterns
Regression analyses between pairs of morphological traits indicated a significant association between body size and head size, head size and head dimensions, and head size and
lateral head shape (Table 2). Comparison of regression coefficients among groups indicated significant differences in allometric trajectories between the sexes—but not between
species—for the relationship between head size and body size, and between head size and
lateral head shape (Table 2). Specifically, males of both species showed a more rapid
increase of head size with increasing body size, and a steeper modification of lateral head
shape with increasing head size (Fig. 4a, b). Additionally, there was a significant difference between species within the same sex in the regression intercept of head length and
height with head size (Table 2), where both sexes of P. bocagei presented relatively shorter
but higher heads than P. hispanica type 1A.
Considering form-function relationships, all biometric traits were shown to significantly
influence bite force (Table 3). A difference in allometric trajectories between the sexes
123
Evol Ecol (2012) 26:825–845
833
Fig. 2 Variation across groups in biometric traits (in mm) and bite force (in N). Points represent means and
vertical bars denote 95% confidence intervals. SVL snout-vent length, HS head size, HH head height, PBF
P. bocagei females, PBM P. bocagei males, PHF P. hispanica females, PHM P. hispanica males
existed for the relationship between bite force and body size (SVL), where males of both
species presented slightly higher slopes than conspecific females (Table 3, Fig. 5a). The
regression slope between bite force and head size was uniform across groups, but males
were found to bite harder than females for the same head size, which was also the case for
individuals of both sexes of P. hispanica type 1A as compared to those of P. bocagei
(Table 3, Fig. 5b). Finally, a significant association was observed between lateral head
shape and bite force, with a significant interaction between bite and sex (ANCOVA:
df = 7, 116; P values for SP = 3.51 9 10-22, SEX = 2.97 9 10-21, bite = 1.12 9 10-5,
SEX 9 bite = 0.027; all other effects P [ 0.1). These results indicated that different
structural relationships exist between lateral head shape and bite force in both sexes (but
not species). In fact, in both species, a tight relationship exists between lateral head shape
and bite force in males, whereas regression slopes are not significantly different from zero
in females (Fig. 5c).
Model selection
Model selection indicated that the model that best described variation in bite force when
examining all groups together included body size (SVL), head size (HS) and head width
123
834
Evol Ecol (2012) 26:825–845
Fig. 3 Spline graphs depicting differences in lateral head shape as quantified through geometric
morphometric methods between species and sexes. a Shape difference between species in males; b: shape
difference between species in females; c: sexual dimorphism in P. bocagei; d: sexual dimorphism in P.
hispanica. Thick black lines represent the vectors of change between the compared shapes and comparisons
are always from P. bocagei to P. hispanica and from females to males. Shape patterns are exaggerated fivefold to enhance visualization
(HW) (Table 4). However, model selection results varied extensively across groups, with
different variables being selected in different species and sexes. Globally, head size, head
width and mouth opening were the most important variables across datasets (Table 4).
Dietary differentiation
Of the 124 lizards examined for morphology and bite force, we obtained pellets suitable for
diet analyses from 100 individuals, containing a total of 440 prey items (prey per pellet
ranging from one to twelve items). When dietary patterns were evaluated, indices of prey
items revealed relatively low diet variation among groups, with the most frequently used
prey types being Homoptera, Coleoptera, Diptera, Hymenoptera and Araneae. Graphical
comparisons of food use indices revealed that the consumption of Homoptera and Coleoptera was slightly divergent between sexes in P. bocagei and between males of both
species (Fig. 6a, c). By contrast, the sexes of P. hispanica type 1A, and females of both
species, displayed almost identical food use patterns (Fig. 6b, d). Considering functional
prey categories, neither prey hardness (Fig. 6e) nor prey mobility (Fig. 6f) showed an
association with prey consumption by different groups (Hardness: v2 = 6.38, df = 6,
P = 0.38; Mobility: v2 = 8.21, df = 6, P = 0.22). Comparison of prey size for the most
commonly used prey items gave variable results: for Homoptera and Hymenoptera no
significant differences were detected between the four lizard groups (P [ 0.05 for all
effects). By contrast, the size of consumed Coleoptera and Diptera differed between sexes
(Coleoptera: F = 4.501, P = 0.038; Diptera: F = 8.075, P = 0.011), but not between
species (P [ 0.05 in both cases). Inspection of prey sizes revealed that significant effects
of sex were due to P. hispanica type 1A females consuming smaller prey in both cases
(Fig. 6g, h). Mantel tests associating food use with morphology and with bite force were
not significant (P [ 0.1 in all cases), except when Euclidean distances between group
means for logSVL as the X matrix were used (P = 0.033).
123
2.05
0.15
SP 9 SEX
X 9 SP 9 SEX
0.48
0.15
<0.001
0.17
<0.001
0.06
0.32
0.58
0.09
1.05
37.11
<0.001
0.48
0.42
0.35
0.48
0.25
<0.001
0.00
3.16
3.67
1.17
2.32
24.29
2,516.02
F
0.56
0.10
0.08
0.30
0.14
<0.001
<0.001
P
logHW (logHS)
GM shape: lateral head shape as quantified using geometric morphometrics
16.57
X 9 SEX
a
1.93
606.87
SEX
X 9 SP
151.49
1,244.84
<0.001
1,321.52
X
SP
<0.001
F
P
F
P
logHL (logHS)
logHS (logSVL)
0.13
1.19
1.41
2.42
0.06
94.6
1,181.43
F
0.48
0.24
0.21
0.14
0.48
<0.001
<0.001
P
logHH (logHS)
0.09
1.44
2.28
0.52
0.01
5.69
3,826.84
F
0.48
0.21
0.14
0.35
0.56
0.03
<0.001
P
logMO (logHS)
0.70
0.91
1.72
0.81
15.20
16.1
4.35
F
0.82
0.58
0.04
0.70
<0.001
<0.001
<0.001
P
GM shapea (logHS)
Table 2 ANCOVA results for comparisons between species (SP) and sexes (SEX) on different combinations of biometric variables. X: effect of the covariate in the model
(indicated in parenthesis next to the dependent variable); F: the F statistic, P the corresponding P-value after correction for multiple testing using the false discovery rate
procedure. Degrees of freedom are 7 (design) and 116 (residuals) in all comparisons. Significant effects are marked in bold letter
Evol Ecol (2012) 26:825–845
835
123
836
Evol Ecol (2012) 26:825–845
Fig. 4 Allometric relationship between head size and body size (a), lateral head shape as captured by
geometric morphometrics and head size (b), head length and head size (c) and head height and head size (d).
Squares, continuous line: males; circles, dashed line: females, black: P. bocagei; grey: P. hispanica type
1A. See Table 2 for tests of homogeneity of slopes and intercepts between groups
Discussion
Understanding the form-function relationship and its ecological implications is of central
importance for understanding how morphological diversity evolves. Our study shows that
the association between external morphology and performance traits can be quite flexible,
such that different pressures can be reconciled for both ecological and social requirements
to be met. In this system, external morphology is molded under the influence of both sexual
and natural selection, but bite performance does not seem to be constrained due to morphological variation. The application of geometric morphometrics allows an integrated
view of how different head components are modified to accommodate ecological pressures
while maintaining performance, providing a powerful tool for future studies.
As commonly observed in Podarcis wall lizards (Braña 1996; Herrel et al. 1996;
Kaliontzopoulou et al. 2007; Brecko et al. 2008; Kaliontzopoulou et al. 2008), both species
examined here present marked sexual dimorphism in head size and shape. Males of both
species have visibly larger heads than conspecific females (Fig. 2b), a pattern proximately
caused by sexual differences in static allometry of head size on body size, where males
present higher allometric slopes than females (Fig. 4a), as is common in many lacertid
species (Braña 1996; Kratochvı́l et al. 2003; Kaliontzopoulou et al. 2006, 2008). Geometric
morphometrics also indicated sexual dimorphism in lateral head shape: males of both
species present an inflation of the tympanic area as compared to conspecific females
(Table 1; Fig. 3c, d). Interestingly, these patterns also coincide with what has been previously reported for P. carbonelli (Kaliontzopoulou et al. 2008), another member of the P.
hispanica species complex (Kaliontzopoulou et al. 2011). Therefore, when our current
findings are viewed in light of previous work, a maintenance of sexual shape patterns can
be observed across this species complex, although only a phylogenetic comparative study
including more members of this group could confirm this hypothesis. Furthermore, the
relationship between head size and lateral head shape also shows important sexual allometric differentiation, where male head shape is markedly associated to head size, a pattern
which is not observed in females (Fig. 4b). Put together, the above patterns provide
123
0.02
0.38
5.86
0.02
SP 9 SEX
BIOM 9 SP 9 SEX
0.13
0.01
6.91
<0.001
1.57
306.59
SEX
<0.001
<0.001
BIOM 9 SEX
36.69
BIOM 9 SP
432.17
BIOM
SP
1.21
5.40
0.16
0.15
12.09
1.68
1066.82
F
F
P
logHS
logSVL
0.17
0.02
0.34
0.34
<0.001
0.13
<0.001
P
0.74
4.34
0.06
1.01
31.02
12.33
560.77
F
logHL
0.20
0.03
0.37
0.18
<0.001
<0.001
<0.001
P
0.76
1.68
0.87
1.15
11.32
3.90
999.73
F
logHW
0.2
0.13
0.19
0.17
<0.001
0.04
<0.001
P
5.16
8.45
0.10
0.09
31.83
43.41
538.86
F
logHH
0.02
<0.001
0.36
0.36
<0.001
<0.001
<0.001
P
1.84
10.92
2.05
0.01
17.77
0.03
872.13
F
logMO
0.13
<0.001
0.11
0.39
<0.001
0.38
<0.001
P
Table 3 Results obtained from linear models on bite force, with species (SP) and SEX as categorical predictors and different morphological predictors (BIOM) as covariates
(specified in columns). F the F-statistic, P: the corresponding P-value after correction for multiple testing using the false discovery rate procedure. Degrees of freedom are 7
(design) and 116 (residuals) in all comparisons. Significant effects are marked in bold letter
Evol Ecol (2012) 26:825–845
837
123
838
Evol Ecol (2012) 26:825–845
Fig. 5 Allometric relationship between morphological traits and bite force. Squares, continuous line:
males; circles, dashed line: females; black: P. bocagei; grey: P. hispanica type 1A. See Table 3 for tests of
homogeneity of slopes and intercepts between groups
Table 4 Model selection statistics on different models of bite force including linear combinations of
morphological measurements for the complete dataset (all) and for each group separately (PBF P. bocagei
females, PBM P. bocagei males, PHF P. hispanica females, PHM P. hispanica males). Regression coefficients for the best model (BM) based on Akaike’s Information Criterion are presented below each predictor. R2 adjusted R2, AICc: Akaike’s Information Criterion corrected for small samples. VI: variable
importance as the sum of Akaike weights for each variable across the set of models with deltaAICc \ 4
(Burnham and Anderson 2002). The two most important variables for each dataset are indicated in bold
letter
All
BM
Intercept
logSVL
-2.31
-0.51
2.29
0.91
0.92
VI
PBF
BM
-2.94
VI
PBM
BM
BM
BM
VI
logHW
0.14
logHH
logMO
1
0.21
0.23
0.33
0.20
0.34
1.56
3.39
0.09
0.81
0.27
0.13
0.29
0.16
0.19
0.14
0.14
1
0.37
0.23
1.56
-1.87
VI
PHM
logHL
-2.92
VI
PHF
logHS
0.83
1.88
0.16
0.68
0.08
0.30
2.44
-2.63
2.24
0.22
0.63
1.26
0.65
R2
AICc
0.9
-290.99
0.88
-77.32
0.83
-76.58
0.53
-67.42
0.76
-70.58
0.27
indirect morphological evidence to the action of sexual selection on head relative size and
shape (Andersson 1994; Bonduriansky and Day 2003; Bonduriansky 2007).
Focusing on interspecific patterns, head size also differs between species, with
P. bocagei having bigger heads than P. hispanica type 1A, although this pattern is only
significant for males (Fig. 2b). Both species are also significantly differentiated in head
shape, as indicated by both linear and geometric morphometrics (Table 1, 2). Both patterns
of lateral head shape (deformation grids, Fig. 3a, b) and ANCOVA on linear head
dimensions with head size as a covariate (Fig. 4c, d) indicate that, in both sexes, P. bocagei
has a relatively shorter and higher head, with a more robust built, as compared to
P. hispanica type 1A. The morphological differentiation observed between P. bocagei and
P. hispanica type 1A has been related to the marked microhabitat divergence between both
species (Sá-Sousa 2001). The smaller and flatter head of P. hispanica type 1A likely
reflects this species’ preference for inhabiting saxicolous habitats, in contrast to the more
ground-dwelling habits of P. bocagei (Sá-Sousa et al. 2002), a pattern typical of rockdwelling lacertids (Arnold 1973, 1987). Interestingly, both linear head dimensions and
123
Evol Ecol (2012) 26:825–845
839
Fig. 6 Dietary patterns observed across the four examined groups. a–d Scatter-plots comparing group use
indices for each of the prey types consumed between both sexes in P. bocagei (a) and P. hispanica type 1A
(b), as well as between species in males (c) and females (d). The continuous line represents a slope of 1, i.e.
equal use of food resources for the compared groups. Ain Indeterminate arthropods, Ara Araneida, Col
Coleoptera, Der Dermaptera, Dip Diptera, Het Heteroptera, Hom Homoptera, Hym Hymenoptera, Lar
Larvae, Lep Lepidoptera, Mec Mecoptera. PBF P. bocagei females, PBM P. bocagei males, PHF P.
hispanica type 1A females, PHM P. hispanica males type 1A. e–f: Percentage of functional prey types
consumed by each group according to prey hardness (e) and mobility (f). SED sedentary, W/J walking/
jumping, FLY flying. g–h Prey size patterns for prey types in which significant differences between groups
were detected (e: Coleoptera, f: Diptera; see ‘‘Results’’)
lateral head shape show similar patterns of static allometry with head size in both species, a
fact that may be indicative of a preserved head size-shape relationship across species, but
once again only a comparison including more species can confirm this hypothesis.
Intriguingly, different studies of head shape-size allometry in lacertids have given different
results. While relative uniformity is observed across species in some cases (Kaliontzopoulou et al. 2008; present study), extensive variation has been reported in others
(Ljubisavljević et al. 2010) and ontogenetic variation has been recently suggested to be a
major source of shape variability in Podarcis lizards (Piras et al. 2011), pointing to a
potentially very fruitful field for future investigation.
But how are the above intersexual and interspecific morphological patterns associated
with bite performance? Our results show that making predictions about performance based
on morphological information alone is not straight-forward; for while significant functional
sexual dimorphism exists in both species examined, bite force does not differ between
species when considering individuals of the same sex (Table 1). Despite their smaller and
visibly flatter heads, males of P. hispanica type 1A bite equally hard as those of P. bocagei
(Fig. 2d). Thus, although microhabitat divergence between species is reflected in their
general head shape, this does not constrain bite performance. Future studies using phylogenetic comparative methods to investigate morphological and performance variation in
this group of lizards would shed more light into how natural selection associated to
microhabitat use may contribute in shaping phenotypic divergence. Importantly, morphological patterns support the hypothesis that an interaction between sexual and natural
selection may occur in this system. Considering variation in head height, a head dimension
that is frequently mechanically constrained in saxicolous lizards (Arnold 1973, 1987,
1998), we observe a significant interaction term (Table 1), indicating that sexual
123
840
Evol Ecol (2012) 26:825–845
dimorphism varies across species, P. bocagei being more dimorphic than P. hispanica type
1A (Fig. 2c). A potential explanation for this pattern is that natural selection associated to
microhabitat use and predator escape is limiting the optimal increase of head height in P.
hispanica type 1A males under the effect of sexual selection, as has been observed in other
lizards (Vitt et al. 1997; Stuart-Fox and Moussalli 2007). However, our results indicate that
both ecological and social demands can be fulfilled in this system, by modification of
morphology-performance relationships across species.
Model evaluation procedures indicate that head size, head width and mouth opening
are the main determinants of bite performance in our system (Table 4). Head size and
width have been previously shown to be good predictors of bite force and in many cases
explains variation in bite performance among individuals (Herrel et al. 2001a; Lappin
et al. 2006; Huyghe et al. 2009). However, model evaluation procedures also show that
the morphological variables most correlated with bite performance vary extensively
across species and sexes, thus indicating that this is a highly flexible and dynamic
system. In this context, our study indicates that the variation observed across different
lizard taxa concerning the external morphological variables that most highly correlate
with bite force and are thus considered good performance predictors may be due to
variation in ecological and social characteristics across these groups. We must also point
out that, at the proximate level, functional similarity can also be achieved by modification of the jaw muscle insertion points, and/or the orientation and length of muscles
(Herrel et al. 1996, 1998a, 2001a). Thus, a detailed study of bite biomechanics and
anatomy of the jaw musculature in these lizards would enable one to tease apart how
different morphological traits and structures might compensate among them to produce
similar performance functionality (Alfaro et al. 2004; Toro et al. 2004; Alfaro et al.
2005).
Nevertheless, a more detailed and complete image of how head shape varies across
species and sexes is provided by the use of geometric morphometric methods, which allow
a better understanding of how the flatter head of P. hispanica type 1A may be compensated
for in terms of bite force. Indeed, deformation grids show that although individuals of both
sexes of P. hispanica type 1A are relatively flatter than those of P. bocagei, they also
present an amplification of the tympanic area (Fig. 3a, b). This is achieved through the
posterior displacement of the ear opening (landmarks 10 and 11, see Fig. 1) and by the
anterior displacement of the posterior limit of the mouth (landmark 8). As is true for sexual
dimorphism patterns (see above), such a shape modification provides more space for the
jaw adductor muscle, potentially enhancing bite force in P. hispanica type 1A. This
example nicely illustrates how geometric morphometric techniques can improve the study
of biomechanical systems. Were only linear biometric traits included in this study, this
kind of shape modification, relevant for understanding the observed patterns of functional
variation, would have been missed (see also Adams and Rohlf 2000). Further, we also find
a direct association between geometric lateral head shape and bite force, significant only in
males and with a common slope between species (Fig. 5c). In this context, head shape and
bite force seem to be integrated through the influence of head size, which scales allometrically between sexes with head shape (Fig. 5c). Thus, allometric head shape modification in males likely increases the performance advantages obtained through a relatively
bigger head, further augmenting bite force. This observation is also consistent with biomechanical predictions, since an amplification of the tympanic area provides more space
for a more developed set of jaw adductor muscles (Herrel et al. 1996), and lends further
support to the importance of sexual selection in shaping morphological patterns by
enhancing bite force in males.
123
Evol Ecol (2012) 26:825–845
841
Based on the observed morphological and performance differentiation, we may also
expect a dietary divergence, particularly between sexes, in respect to the type and size of
prey consumed. In fact, head size and shape variation have been frequently associated with
dietary divergence (Adams and Rohlf 2000; Herrel et al. 1996, 2001d; Verwaijen et al.
2002; Maerz et al. 2006; Vincent and Herrel 2007; Brecko et al. 2008). Nonetheless, our
analysis of dietary divergence provided little support for this prediction, contradicting
previous observations in other lacertid species (i.e. Vincent and Herrel 2007 and references
therein). For instance, prey consumption did not vary much across the four SP 9 SEX
groups examined, although some localized differences did exist. Specifically, male
P. bocagei consumed relatively less Homoptera and more Coleoptera than conspecific
females or male P. hispanica type 1A (Fig. 6a, c), while female P. hispanica type 1A
consumed significantly smaller Coleoptera and Diptera than the remaining groups (Fig. 6g,
h). These results indicate a tendency towards differential prey consumption by some
groups, the detection of which might be restricted by low statistical power due to low
sample sizes in terms of retrieved prey items per pellet, or the examination of a single
sampling event, not taking seasonal variation in diet into account. Previous studies have
drawn the attention to the fact that in most cases field dietary data do not offer sufficient
resolution to assess ecological divergence (Herrel et al. 2001b). In fact, our results rather
indicate a significant association between dietary divergence and differences between
groups in total body size, possibly pointing to the influence of other factors, such as
foraging tactics (McBrayer 2004; Schwenk 2000), predation pressure and time-energy
budgets during prey consumption, or even microhabitat use (Arnold 1987), which may be
relatively more influential on prey utilization (e.g. Diaz 1995; Herrel et al. 2001d;
Carretero 2004).
Together, our findings indicate that although body and head size are tightly linked to
bite performance in these two Podarcis species, in accordance with previous studies, they
are rather secondary in explaining the observed functional variation. Instead, head
dimensions and head shape also play an important role in determining bite performance,
thereby providing the potential for the maintenance of equivalent performance under
different head configurations. In fact, this system provides a nice example of relatively
rapid morphological differentiation (P. bocagei and P. hispanica type 1 diverged
approximately 5.5 MYA, i.e. Kaliontzopoulou et al. 2011), without concordant functional
divergence between two sister groups of lizards. Furthermore, our results indicate that
group differentiation in head size and shape and bite force is not reflected in the dietary
similarity neither between species nor between sexes, suggesting that the evolutionary
causation of the observed morphological and performance patterns should be sought
elsewhere. Given the close relationship between head morphology and bite performance
observed across sexes, we conclude that the form-function association in lizards is tightly
linked to the observed sexual dimorphism in both aspects, with sexual selection potentially
being the underlying evolutionary force, as has been proven for other lizard groups (Husak
et al. 2006a; Lailvaux and Irschick 2006). However, such a hypothesis can only be definitely confirmed by directly examining the reproductive fitness of male individuals and
associating it to both head morphology and bite performance (Lappin and Husak 2005;
Irschick et al. 2007), as has been done for the form-locomotor performance complex
(Husak et al. 2006b; Husak and Fox 2008).
Acknowledgments We thank Catarina Rato and Fatima Jorge for help during fieldwork and experimental
treatments. Anthony Herrel kindly provided the force transducer used for bite-force measurements. Two
anonymous reviewers provided insightful comments that aided in improving a previous version of the
123
842
Evol Ecol (2012) 26:825–845
manuscript. AK, AvdM and AP were supported by post-doctoral grants (SFRH/BPD/68493/2010, SFRH/
BPD/48042/2008 and SFRH/BPD/26546/2006 respectively), all from Fundação para a Ciência e Tecnologia
(FCT, Portugal), and DCA was supported in part by NSF grant DEB-0446758. Capture and handling of the
animals used were performed under a scientific permit provided by Instituto para a Conservação da Natureza
e Biodiversidade (ICNB, Portugal).
References
Adams DC (2004) Character displacement via aggressive interference in Appalachian salamanders. Ecology 85:2664–2670
Adams DC, Nistri A (2010) Ontogenetic convergence and evolution of foot morphology in European cave
salamanders (Family: Plethodontidae). BMC Evol Biol 10(216):1–10
Adams DC, Rohlf FJ (2000) Ecological character displacement in Plethodon: biomechanical differences
found from a geometric morphometric study. Proc Nat Acad Sci USA 97:4106–4111
Adams DC, Rohlf FJ, Slice DE (2004) Geometric morphometrics: ten years of progress following the
‘revolution’. Ital J Zool 71:5–16
Alfaro ME, Bolnick DI, Wainwright PC (2004) Evolutionary dynamics of complex biomechanical systems:
an example using the four-bar mechanism. Evolution 58:495–503
Alfaro ME, Bolnick DI, Wainwright PC (2005) Evolutionary consequences of many-to-one mapping of jaw
morphology to mechanics in labrid fishes. Am Nat 165:E140–E154
Andersson M (1994) Sexual selection: monographs in behavior and ecology. Princeton University Press,
Princeton
Arnold EN (1973) Relationships of the Palearctic lizards assigned to the genera Lacerta, Algyroides and
Psammodromus (Reptilia: Lacertidae). Bull Br Mus 25:289–366
Arnold SJ (1983) Morphology, performance and fitness. Am Zool 23:347–361
Arnold EN (1987) Resource partition among lacertid lizards in southern Europe. J Zool Lond (B) 1:739–782
Arnold EN (1998) Cranial kinesis in lizards: variations, uses and origins. Evol Biol 30:323–357
Bonduriansky R (2007) Sexual selection and allometry: a critical reappraisal of the evidence and ideas.
Evolution 61:838–849
Bonduriansky R, Day T (2003) The evolution of static allometry in sexually selected traits. Evolution
57:2450–2458
Bookstein FL (1991) Morphometric tools for landmark data: geometry and biology. Cambridge University
Press, Cambridge
Braña F (1996) Sexual dimorphism in lacertid lizards: male head increase vs female abdomen increase?
Oikos 75:511–523
Brecko J, Huyghe K, Vanhooydonck B, Herrel A, Grbac I (2008) Functional and ecological relevance of
intraspecific variation in body size and shape in the lizard Podarcis melisellensis (Lacertidae). Biol J
Linn Soc 94:251–264
Bruner E, Constantini D (2007) Head morphological variation in Podarcis muralis and Podarcis sicula: a
landmark-based approach. Amphib Reptilia 28:566–573
Burnham KP, Anderson DR (2002) Model selection and multi-model inference: a practical-theoretic
approach, 2nd edn. Springer, New York
Butler MA, Losos JB (2002) Multivariate sexual dimorphism, sexual selection, and adaptation in Greater
Antillean Anolis lizards. Ecol Monogr 72:541–559
Butler MA, Sawyer SA, Losos JB (2007) Sexual dimorphism and adaptive radiation in Anolis lizards. Nature
447:202–205
Carretero MA (2004) Form set menu to a la carte. Linking issues in trophic ecology of Mediterranean
lizards. Ital J Zool Supp 2:121–133
Carretero MA (2008) An integrated assessment of the specific status in a group with complex systematics:
the Iberomaghrebian lizard genus Podarcis (Squamata, Lacertidae). Integr Zool 4:247–266
Carretero MA, Llorente GA (2001) What are they really eating? Stomach versus intestine as sources of diet
information in lacertids. In: Vicente L, Crespo EG (eds) Mediterranean basin lacertid lizards: a biological approach. ICN, Lisbon, pp 105–112
Cooper WE Jr., Vitt LJ (1989) Sexual dimorphism of head and body size in an iguanid lizard: paradoxical
results. Am Nat 133:729–735
Diaz JA (1995) Prey selection by lacertid lizards: a short review. Herp J 5:245–251
Drake AG, Klingenberg CP (2008) The pace of morphological change: historical transformation of skull
shape in St Bernard dogs. Proc R Soc B 275:71–76
123
Evol Ecol (2012) 26:825–845
843
Harris DJ, Sá-Sousa P (2002) Molecular phylogenetics of Iberian Wall Lizards (Podarcis): is Podarcis
hispanica a species complex? Mol Phylogenet Evol 23:75–81
Herrel A, O’Reilly JC (2006) Ontogenetic scaling of bite force in lizards and turtles. Phys Biochem Zool
79:31–42
Herrel A, Van Damme R, de Vree F (1996) Sexual dimorphism of head size in Podarcis hispanica atrata:
testing the dietary divergence hypothesis by bite force analysis. Neth J Zool 46:253–262
Herrel A, Aerts P, De Vree F (1998a) Ecomorphology of the lizard feeding apparatus: a modelling approach.
Neth J Zool 48:1–25
Herrel A, Aerts P, De Vree F (1998b) Static biting in lizards: functional morphology of the temporal
ligaments. J Zool Lond 244:135–143
Herrel A, de Grauw E, Lemos-Espinal JA (2001a) Head shape and bite performance in xenosaurid lizards.
J Exp Zool 290:101–107
Herrel A, Meyers JJ, Vanhooydonck B (2001b) Correlations between habitat use and body shape in a
phrynosomatid lizard (Urosaurus ornatus): a population-level analysis. Biol J Linn Soc 74:305–314
Herrel A, Meyers J, Nishikawa KC, De Vree F (2001c) The evolution of feeding motor patterns in lizards:
modulatory complexity and possible constraints. Am Zool 41:1311–1320
Herrel A, Van Damme R, Vanhooydonck B, De Vree F (2001d) The implications of bite performance for
diet in two species of lacertid lizards. Can J Zool 79:662–670
Herrel A, McBrayer LD, Larson PM (2010) Functional basis for sexual differences in bite force in the lizard
Anolis carolinensis. Biol J Linn Soc 91:111–119
Husak JF, Fox SF (2008) Sexual selection on locomotor performance. Evol Ecol Res 10:213–228
Husak JF, Lappin AK, Fox SF, Lemos-Espinal JA (2006a) Bite-force performance predicts dominance in
male venerable collared lizards (Crotaphytus antiquus). Copeia 2006:301–306
Husak JF, Fox SF, Lovern MB, van den Bussche RA (2006b) Faster lizards sire more offspring: sexual
selection on whole-animal performance. Evolution 60:2122–2130
Husak JF, Lappin AK, van den Bussche RA (2009) The fitness advantage of a high-performance weapon.
Biol J Linn Soc 96:840–845
Huyghe K, Vanhooydonck B, Scheers H, Molina-Borja M, Van Damme R (2005) Morphology, performance
and fighting capacity in male lizards, Gallotia galloti. Funct Ecol 19:800–807
Huyghe K, Herrel A, Adriaens D, Tadić Z, Van Damme R (2009) It is all in the head: morphological basis
for differences in bite force among colour morphs of the Dalmatian wall lizard. Biol J Linn Soc
96:13–22
Irschick DJ, Herrel A, Vanhooydonck B, Van Damme R (2007) A functional approach to sexual selection.
Funct Ecol 21:621–626
Irschick DJ, Meyers JJ, Husak JF, Le Galliard JF (2008) How does selection operate on whole-organism
functional performance capacities? A review and synthesis. Evol Ecol Res 10:177–196
Kaliontzopoulou A, Carretero MA, Llorente GA (2006) Patterns of shape and size sexual dimorphism in a
population of Podarcis hispanica* (Reptilia: Lacertidae) from NE Iberia. In: Corti C, Lo Cascio P,
Biaggini M (eds) Mainland and insular lacertid lizards: a mediterranean perspective. Firenze University Press, Firenze, pp 73–89
Kaliontzopoulou A, Carretero MA, Llorente GA (2007) Multivariate and geometric morphometrics in the
analysis of sexual dimorphism variation in Podarcis lizards. J Morphol 268:152–165
Kaliontzopoulou A, Carretero MA, Llorente GA (2008) Head shape allometry and proximate causes of head
sexual dimorphism in Podarcis lizards: joining linear and geometric morphometrics. Biol J Linn Soc
93:111–124
Kaliontzopoulou A, Carretero MA, Llorente GA (2010) Intraspecific ecomorphological variation: linear and
geometric morphometrics reveal habitat-related patterns within Podarcis bocagei wall lizards. J Evol
Biol 23:1234–1244
Kaliontzopoulou A, Pinho C, Harris DJ, Carretero MA (2011) When cryptic diversity blurs the picture: a
cautionary tale from Iberian and North African Podarcis wall lizards. Biol J Linn Soc 103:779–800
Kingsolver J, Huey RB (2003) Introduction: the evolution of morphology, performance, and fitness. Integr
Comp Biol 43:361–366
Kratochvı́l L, Frynta D (2002) Body size, male combat and the evolution of sexual dimorphism in eublepharid geckos (Squamata: Eublepharidae). Biol J Linn Soc 76:303–314
Kratochvı́l L, Fokt M, Rehák I, Frynta D (2003) Misinterpretation of character scaling: a tale of sexual
dimorphism in body shape of common lizards. Can J Zool 81:1112–1117
Lailvaux SP, Irschick DJ (2006) A functional perspective on sexual selection: insights and future prospects.
Anim Behav 72:263–273
Lailvaux SP, Irschick DJ (2007) The evolution of performance-based male fighting ability in Caribbean
Anolis lizards. Am Nat 170:573–586
123
844
Evol Ecol (2012) 26:825–845
Lappin AK, Husak JF (2005) Weapon performance, not size, determines mating success and potential
reproductive output in the collared lizard (Crotaphytus collaris). Am Nat 166:426–436
Lappin AK, Hamilton PS, Sullivan BK (2006) Bite-performance and head shape in a sexually dimorphic
crevice-dwelling lizard, the common chuckwalla [Sauromalus ater (=obesus)]. Biol J Linn Soc
88:215–222
Legendre P, Legendre L (1998) Numerical ecology, 2nd English edn. Elsevier, Amsterdam
Ljubisavljević K, Urosević A, Aleksić I, Ivanović A (2010) Sexual dimorphism of skull shape in a lacertid
lizard species (Podarcis spp., Dalmatolacerta sp., Dinarolacerta sp.) revealed by geometric morphometrics. Zoology 113:168–174
Losos JB, Butler MA, Schoener TW (2003) Sexual dimorphism in body size and shape in relation to habitat
use among species of Caribbean Anolis lizards. In: Fox SF, McCoy JK, Baird TA (eds) Lizard social
behaviour. John Hopkins University Press, Baltimore, pp 356–380
Maerz JC, Myers EM, Adams DC (2006) Trophic polymorphism in a terrestrial salamander. Evol Ecol Res
8:23–35
McBrayer LD (2004) The relationship between skull morphology, biting performance and foraging mode in
Kalahari lacertid lizards. Zool J Linn Soc 140:403–416
Measey GJ, Hopkins K, Tolley KA (2009) Morphology, ornaments and performance in two chameleon
ecomorphs: is the casque bigger than the bite? Zoology 112:217–226
Metzger KA, Herrel A (2005) Correlations between lizard cranial shape and diet: a quantitative, phylogenetically informed analysis. Biol J Linn Soc 86:433–466
Mosimann JE (1970) Size allometry: size and shape variables with characterizations of the lognormal and
generalized gamma distributions. J Am Stat Assoc 65:930–945
Perrin N, Travis J (1992) On the use of constraints in evolutionary biology and some allergic reactions to
them. Funct Ecol 6:361–363
Perry G, Pianka ER (1997) Animal foraging: past, present and future. Trends Ecol Evol 12:360–364
Pianka ER (1973) The structure of lizard communities. Annu Rev Ecol Syst 4:53–74
Pinho C, Ferrand N, Harris DJ (2006) Reexamination of the Iberian and North African Podarcis (Squamata:
Lacertidae) phylogeny based on increased mitochondrial DNA sequencing. Mol Phylogenet Evol
38:266–273
Piras P, Salvi D, Ferrara G, Maiorino L, Delfino M, Pedde L, Kotsakis T (2011) The role of postnatal
ontogeny in the evolution of phenotypic diversity in Podarcis lizards. J Evol Biol (in press)
Preest MR (1994) Sexual size dimorphism and feeding energetics in Anolis carolinensis: why do females
take smaller prey than males? J Herpetol 28:292–298
Raia P, Guarino FM, Turano M, Polese G, Rippa D, Carotenuto F, Monti DM, Cardi M, Fulgione D (2010)
The blue lizard spandrel and the island syndrome. BMC Evol Biol 10:289
Rivera G (2008) Ecomorphological variation in shell shape of the freshwater turtle Pseudemys concinna
inhabiting different aquatic flow regimes. Integr Comp Biol 48:769–787
Rohlf FJ (2005) tpsDig, digitize landmarks and outlines, version 2.04. Department of Ecology and Evolution, State University of New York at Stony Brook, Stony Brook
Rohlf FJ (2008) tpsRelw, relative warps analysis, version 1.46. Department of Ecology and Evolution, State
University of New York at Stony Brook, Stony Brook
Rohlf FJ (2009) tpsRegr, version 1.37. Department of Ecology and Evolution, State University of New York
at Stony Brook, Stony Brook
Rohlf FJ, Marcus LF (1993) A revolution in morphometrics. Trends Ecol Evol 8:129–132
Rohlf FJ, Slice DE (1990) Extensions of the Procrustes method for the optimal superimposition of landmarks. Syst Zool 39:40–59
Sá-Sousa P (2001) A controversa sistemática das lagartixas do género Podarcis Wagler, 1830 (Sauria,
Lacertidae) em Portugal. PhD thesis, University of Lisbon, Lisbon
Sá-Sousa P, Vicente L, Crespo EG (2002) Morphological variability of Podarcis hispanica (Sauria:
Lacertidae) in Portugal. Amphib Reptilia 23:55–69
Schwenk K (2000) Feeding in lepidosaurs. In: Schwenk K (ed) Feeding: form, function and evolution in
tetrapod vertebrates. Academic Press, San Diego, pp 175–291
Slice DE (1999) Morpheus et al.: software for morphometric research. Department of Ecology and Evolution, State University of New York, Stony Brook
Slice DE (2005) Modern morphometrics. In: Slice DE (ed) Modern morphometrics in physical anthropology. Kluwer, New York, pp 1–46
Stamps J (1983) Sexual selection, sexual dimorphism and territoriality. In: Huey RB, Pianka ER, Schoener
TW (eds) Lizard ecology: studies of a model organism. Harvard University Press, Cambridge,
pp 169–204
123
Evol Ecol (2012) 26:825–845
845
Stuart-Fox D, Moussalli A (2007) Sex-specific ecomorphological variation and the evolution of sexual
dimorphism in dwarf chameleons (Bradypodion spp.). J Evol Biol 20:1073–1081
R Development Core Team (2010) R: A language and environment for statistical computing. R Foundation
for Statistical Computing, Vienna. http://www.R-project.org
Toro E, Herrel A, Irschich D (2004) The evolution of jumping performance in Caribbean Anolis lizards:
solutions to biomechanical trade-offs. Am Nat 163:844–856
Vanhooydonck B, Van Damme R (1999) Evolutionary relationships between body shape and habitat use in
lacertid lizards. Evol Ecol Res 1:785–805
Verwaijen D, Van Damme R, Herrel A (2002) Relationships between head size, bite force, prey handling
efficiency and diet in two sympatric lacertid species. Funct Ecol 16:842–850
Vincent SE, Herrel A (2007) Functional and ecological correlates of ecologically-based dimorphisms in
squamate reptiles. Integr Comp Biol 47:172–188
Vitt LJ, Caldwell JP, Zani PA, Titus TA (1997) The role of habitat shift in the evolution of lizard morphology: evidence from tropical Tropidurus. Proc Nat Acad Sci USA 94:3828–3832
Wainwright PC (2007) Functional versus morphological diversity in macroevolution. Annu Rev Ecol Evol
Syst 38:381–401
Walker JA (2007) A general model of functional constraints on phenotypic evolution. Am Nat 170:681–689
Young MT, Brusatte SL, Ruta M, De Andrade MB (2010) The evolution of Metriorhynchoidea (Mesoeucrocodylia, Thalattosuchia): an integrated approach using geometric morphometrics, analysis of disparity, and biomechanics. Zool J Linn Soc 158:801–859
123